Study Of Optical Fiber Cavity Ring-down Sensing Method Based On Frequency-shifted Interferometry

Fiber cavity ring-down spectroscopy technique has many advantages by connecting cavity ring-down spectroscopy(CRDS) with optical fiber sensing. It has caused great attention among researchers in recent years, for measuring many key parameters in the interdisciplinary field. However, the sensitivity and high performance cost ratio of the system got in trouble, especially in the filed of reflecting CRDS superiority of the gas sensing, because of the high inherent cavity loss, necessity of fast detectors. By combining both the frequency-shifted interferometry(FSI) and fiber cavity ring-down(CRD), in this thesis, we systematic studied the sensing method of FSI-CRD. The main contents and results of the thesis include the following:(1) We analyzed the cavity ring-down research status in the domestic and foreign, introduced the advantages and disadvantages of mirror CRDS, fiber CRDS, and FSI-CRD, respectively;(2) We studied the principle of the continuous-wave FSI-CRD sensing technology and the method of data processing;(3) We first applied FSI-CRD to evanescent-field sensing by placing the sensor under uniform magnetic field environment, and successfully measured the magnetic field in the fiber cavity, which was found to be 0.00105±0.00003dB/Gs in the approximately line segment. The threshold point, rising period and saturation point of the curve were explained.(4) A novel method for gas sensing with FSI-CRD based on dual-wavelength differential absorption was proposed. The measurement resolution of 7.8125%/dB was achieved in the linear relationship between gas absorption loss and ethylene concentration. The linear relationship was good agreement with theories. The standard deviations of the cavity loss with single wavelength are greater than that of dual-wavelength absorption loss, which indicates dual-wavelength differential absorption method can efficiently eliminate the influence of various external factors. After repeating data of 1.0% ethylene concentration over a 70-minute period, the relative deviation of the concentration over this period is less than ±0.29%, indicating that the system has a good stability and repeatability.After the measurement of magnetic field and gas sensing, the results indicate FSI-CRD posses the ability detecting interdisciplinary parameters and has better technical performance index, especially suitable for detecting toxic or harmful gases, such as CO, CH4, HS and so on. There are very important scientific significance and practical values to explore a novel technology of optical fiber sensing.